1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to video coding-decoding (CODEC), and more particularly, to image compression and decompression in which coding efficiency is improved by coding a plurality of pictures through cross-reference between the pictures.
2. Description of the Related Art
An image signal generally has high redundancy (also called correlation), i.e., has much information that can be reduced. A signal value of a pixel has a high correlation with that of an adjacent pixel. Orthogonal transformation reduces spatial redundancy and prediction coding reduces temporal redundancy. In particular, prediction coding represents a signal value of a pixel as a differential from a signal value of a pixel in a different time, i.e., a signal value of a past or future pixel. The prediction coding includes intraprediction that does not use a prediction memory and interprediction that uses a prediction memory.
The prediction coding is performed on intra (I) or predicted (P) pictures. An I picture is encoded only using information within itself without referencing any frames and is generated in an intraprediction mode (also called an intra mode). At least one I picture is required for a random access in each group of pictures (GOP). All pixels in the I picture are encoded in the intraprediction mode. A P picture is generated through prediction from an I picture or another P picture. In general, all pixels in the P picture are encoded in the intraprediction mode or an interprediction mode (also called an inter mode). In addition, the prediction coding may also be performed on bidirectional predicted (B) pictures generated by bidirectional prediction.
FIGS. 1A and 1B are reference diagrams for explaining a conventional image compression method.
Referring to FIG. 1A, in the conventional image compression method, an I picture is encoded in an intra mode and only a portion of a P picture, which is different from the encoded I picture, is encoded in an inter mode. More specifically, referring to FIG. 1B, all pixels included in the I picture are encoded in units of a slice in the intra mode, but all pixels included in the P picture are encoded in the inter mode or the intra mode by referring to corresponding slices of the I picture. In other words, in a conventional coding method, intracoding is the only coding type performed on an I picture, and other types of pictures such as P or B pictures are coded by referring to the intracoded I picture. Numbers shown in FIG. 1B indicate the order of coding of corresponding slices, and slices of the P picture are coded after all slices of the I picture are coded.